What are the requirements to label a wine as “Primitivo”?

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Dear Dr. Vinny,
In the United States, what are the requirements to label a wine as “Primitivo”?
—John, Ukiah, Calif.
Dear John,
That’s a great question. Genetically, Primitivo and Zinfandel are similar, both clones of a Croatian grape called Crljenak.
But even though some winemakers think of Primitivo and Zinfandel as synonyms, the Alcohol and Tobacco Tax and Trade Bureau (TTB) doesn’t allow them to be used as synonyms. So if you bottle a wine that is 75 percent or more of the Primitivo clone, you can call it “Primitivo,” and if you have a wine that is 75 percent or more of the Zinfandel clone, you can call it “Zinfandel”.
What’s a little confusing is that the European Union recognizes Zinfandel and Primitivo as synonyms, so Italian Primitivos can be labeled as “Zinfandel” and sold here. There was a proposal back in 2002 to allow the names to be used interchangeably in the United States, but it failed to gain support.
—Dr. Vinny